As a Sn-based lead-free solder, a Sn—Ag—Cu based solder, for example, Sn-3.0Ag-0.5Cu, is widely used. In recent years, as a price of Ag rises, a price of the solder containing Ag also rises. Therefore, a solder alloy having a low Ag content, like Sn-1.0Ag-0.7Cu or Sn-0.3Ag-0.7Cu, attracts attention.
If the Ag content in a solder alloy is decreased, a melting point of the solder alloy is increased. In the solder alloy having the low Ag content, it is feared that the strength and reliability of the solder is deteriorated because an amount of an intermetallic compound which is deposited on a joining interface is decreased.
In order to reduce the melting point of the Sn—Ag—Cu based solder and improve the strength and reliability of the solder, it is effective to add Bi thereto. In the field of onboard equipment where a solder with high strength is required, by adding Bi to a Sn based solder containing about 3 math % (hereinafter, refer to as “wt %”) to 5 wt % of Ag to make Bi dissolved into Sn, the mechanical strength of the solder is improved.
However, if Bi is added to the Sn—Ag—Cu based solder, fragile Bi is deposited on an interface between an electrode and the solder, and fatigue lifetime of the joining portion between the electrode and the solder may be shortened. Therefore it is appropriate that a Bi-free solder is used in products in which high reliability and long lifetime are required.
PTLs 1 to 3 disclose a solder paste containing a Bi-free solder alloy powder.
PTL 1 discloses the solder alloy powder containing 1.0 to 4.0 wt % of Ag, 0.4 to 1.0 wt % of Cu, 1 to 8 wt % of Sb, and a balance of Sn. PTL 1 also discloses the solder alloy of the Ag—Cu—Sb—Sn based alloy containing 0.4 wt % or less of at least one element selected from Ni, Co, and Fe.
PTL 2 discloses the solder alloy containing 0.5 to 3.5 wt % of Ag, 0.1 to 2.8 wt % of Cu, 0.2 to 2.0 wt % of Sn, and a balance of Sn.
PTL 3 discloses the solder alloy containing 0.1 to 5 wt % of Ag, 0.1 to 5 wt % of Cu, 10 wt % or less of a transformation delaying element, 10 wt % or less of an oxidation inhibiting element, and a balance of Sn. PTL 3 discloses the example which describes a whisker resistance property is improved by adding combination of 0.01 to 1 wt % of the transformation delaying element and 0.01 wt % of the oxidation inhibiting element to the Sn based solder alloy.